You can build a great-looking product, but the perception of its quality won't be so good if it's not usable. Sumithu Rukshitha describes the 7 principles of user experience that you should be mindful of when testing.
Furthermore, you might want to listen to some advice on Creating test cases from user behaviour.
Since machine learning is becoming a popular solution in more and more companies, it's good to learn what challenges hide there for testers. Fabio Casati gives a great, detailed overview.
You may also want to learn What is A/B Testing and how it works in Machine Learning.
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